Thermotropic liquid crystalline polymers are condensation polymers that have relatively rigid and linear polymer chains so that they melt to form a liquid crystalline phase. Atypical process for producing liquid crystalline aromatic polyesters involves mixing one or aromatic diols and dicarboxylic acids and/or hydroxycarboxylic acids with enough of a carboxylic acid anhydride (e.g., acetic anhydride) to acetylate the hydroxyl groups of the diols and/or hydroxycarboxylic acids present. Once formed, the acetylated monomers are thereafter heated to a high temperature to initiate a condensation reaction in which the monomers are converted to a polymer. To favor a reaction equilibrium that optimizes the production of a high molecular weight polymer, byproducts of the condensation reaction (e.g., acetic acid, phenolic derivatives, etc.) are generally removed. The mixture is eventually heated to a relatively high temperature, typically in latter stages under vacuum, to produce the final liquid crystalline polymer. This is done while the process mixture is a liquid (in the melt).
Due to their high melting temperature and strength, it is often desirable to form liquid crystalline polymers with a higher molecular weight. Unfortunately, such polymers are highly viscous and thus tend to solidify in the reactor vessel used during melt polymerization, thereby making them extremely difficult to remove from the reactor. For this reason, conventional techniques for forming high molecular weight liquid crystalline polymers have normally involved “solid state polymerization.” That is, before the polymer is fully formed (the molecular weight has reached the desired level), the liquid is cooled and then broken into small particles. These particles are heated while in the “solid state” under a stream of inert gas (e.g., nitrogen) or under a vacuum to raise the molecular weight to the desired level. While solid state polymerization techniques can help achieve the desired molecular weight, they can be overly complex and expensive.
As such, a need exists for a method of melt polymerizing a high molecular weight liquid crystalline polymer with a lower melt viscosity.